I often find it best to buy an international money order in the currency
the purchaser uses, making sure to allow for their charges for
*international* delivery (and that they offer this option!) and not their
domestic delivery charges. Credit card companies (don't know about PayPal,
but I doubt they're as flexible as the big credit card companies) will do
it for you, but obtaining an international money order in the appropriate
denomination is simpler and you know when you buy it how much it comes
to in your money. Ummm, barring customs charges at your border, which you
can find out by calling your nearest customs office, but which you
generally have to pay on arrival of the goods.

Getting the currency the company you want to buy from uses is especially
important when ordering *from* a US company when outside the US since US
banks are notoriously slow and expensive at changing foreign money for
their small customers, and even if a small business will agree to accept
foreign money, they naturally will have to charge enough to cover their
bankers' charges.

Also - I always used national postal services when ordering from outside
my country. I have had rather acrimonious conversations with courier
companies when I either forgot to specify or was sent something by a third
party who didn't think about this, and was asked to pay enormous fees for
clearing customs. This is actually a genuine service - companies that
import a lot regularly will hire customs brokers - but can very easily
come to more than the value of the parcel for an individual ordering one
small object. Post offices don't charge this fee; couriers often, if not
always, do.

I almost never order from outside my own country any more. It gets too
expensive.